The making of integrated optical circuits is based on the use of microlithography techniques that allow mass production. A single-mode optical waveguide may be made on a planar substrate through steps of masking and deposition of a narrow strip of material, possibly followed by a step of thermal diffusion. In an integrated optical circuit, such as in an optical fiber, the effect of optical guiding is linked to a difference of refraction index between the optical waveguide and the substrate, the waveguide refraction index being higher than that of the substrate. Various materials may be used for the making of integrated optical circuits, such as III-V semiconductors, silica on silicon, glass or lithium niobate (LiNbO3) or lithium tantalate (LiTaO3). The lithium tantalate and the lithium niobate are particularly interesting materials because they have a Pockels electro-optic effect. By arranging electrodes on either side of the integrated waveguide, it is possible to modulate the waveguide index and thus to modulate the phase of an optical signal propagating in the waveguide. In an integrated optical circuit where the electrodes are separated by about ten microns, the application of a voltage of only a few volts is sufficient to generate an electric field and to induce the desired phase modulation. By comparison, in a conventional optical phase modulator, the electrodes being separated by at least one millimeter, the electric voltage required to generate a same electric field between the electrodes is of several hundreds of volts.
Various technologies of making lithium-niobate integrated optical circuits have been developed: first the titanium (Ti) diffusion technique, then the proton exchange technique. The titanium diffusion technique consists in depositing a titanium strip on the surface of a lithium niobate substrate, then heating the substrate so that the titanium diffuses into the substrate and locally increases the refraction index. The titanium diffusion technique requires a high temperature (900 to 1100° C.). The proton exchange technique consists in placing a birefringent LiNbO3 crystal in an acid bath so as to replace Li+ ions by H+ ions (i.e. protons). The proton exchange technique is performed at a lower temperature than the titanium diffusion. Moreover, the technique of proton exchange on a birefringent LiNbO3 crystal has for effect both to increase the extraordinary index of the crystal, which creates guidance for a polarization according to the extraordinary axis, and to reduce the ordinary index of the crystal, so that a polarization according to the ordinary axis is not guided. In the proton-exchange LiNbO3 circuits, the usual configuration is an X-cut, the X axis of the single-axis birefringent LiNbO3 crystal being perpendicular to the surface of the substrate, while the Y and Z axes of the crystal are parallel to the surface. The waveguide propagation axis is parallel to the Y direction, and the TE mode (“Transverse Electric” mode, i.e. the electric field is parallel to the surface of the substrate) is parallel to the Z direction. In this case, the proton-exchange optical waveguide guides only the TE polarization state, the TM cross-polarization state (“Transverse Magnetic” mode, i.e. the magnetic field is parallel to the surface of the substrate and thus the electric field is perpendicular to the surface of the substrate) propagating freely in the substrate. The lithium-niobate proton-exchange technique thus allows making a polarizer on integrated optical circuit.
Many integrated optical circuits are thus made from lithium niobate: polarizer, phase modulator, Mach-Zehnder interferometer, Y junction, 2×2 coupler or 3×3 coupler. Advantageously, a same optical circuit integrates several functions on a same substrate, which allows improving the compactness and reducing the optical connections. The lithium-niobate proton-exchange integrated optical circuits find applications in particular in the optical fiber gyroscopes.
In an integrated optical circuit, an input beam is generally coupled to an end of an optical waveguide through an optical fiber. However, only certain modes (for example, polarization mode) are guided by the waveguide, the other modes propagating freely in the substrate. Moreover, if the core of the fiber is not perfectly aligned with the waveguide of the integrated optical circuit, a part of the incident light beam may be coupled in the substrate and propagate outside the waveguide. A part of the light that is not guided by the waveguide may be reflected by total internal reflection on one or several faces of the substrate. In fine, a part of this non-guided light may be coupled to an output optical fiber facing another end of the waveguide. The non-guided light may thus disturb the operation of an integrated optical circuit. For example, in the case of a lithium-niobate proton-exchange polarizer, the polarization rejection rate may be affected by the coupling of light transmitted in a non-guided way by the substrate. Likewise, in the case of a 2×2 or 3×3 coupler, the non-guided light may be coupled via the substrate from an input to an output of the integrated optical circuit.
FIG. 1 schematically shows a perspective view of an integrated optical circuit according to the prior art. The integrated optical circuit comprises a planar substrate 10. By convention in the present description, the substrate 10 comprises an input face 1, an output face 2, a lower face 4, an upper face 3 and two side faces 5. The lower face 4 and the upper face 3 extend between the input face 1 and the output face 2. The lower face 4 and the upper face 3 are opposite to each other. Preferably, the lower face 4 and the upper face 3 are planar and parallel to each other. Likewise, the side faces 5 are planar and parallel to each other and extend between the input face 1 and the output face 2. The input face 1 and the output face 2 of the substrate may also be planar and polished, but they are preferably cut with an inclination angle so as to avoid the spurious back-reflections at the ends of the waveguide. The substrate 10 comprises a rectilinear optical waveguide 6 that extends between a first end 7 on the input face 1 and a second end 8 on the output face 2. By convention, the waveguide 6 is nearer the upper face 3 than the lower face 4. In the case of a lithium-niobate proton-exchange polarizer, the optical waveguide 6 is located below the upper face 3 of the substrate and extends in a plane parallel to the upper face 3. The optical waveguide 6 may be delimited by the upper face or be buried just below this upper face. In other types of IOC, the waveguide 6 may be deposited on the upper surface 3 or may extend inside the substrate, for example in an plane parallel to the upper face 3, half the way between the lower face 4 and the upper face 3. An input optical fiber 20 and an output optical fiber 30 are optically coupled to the first end 7 and the second end 8, respectively, of the waveguide 6. The input optical fiber 20 transmits an optical beam in the integrated optical circuit. A part of the optical beam is guided by the waveguide. The guided beam 12 propagates up to the end 8 of the waveguide 6 facing the output fiber 30. Due to a mode mismatch between the core of the optical fiber 20 and the integrated waveguide 6, another part of the beam is not coupled in the waveguide and propagates freely in the substrate 10. A non-guided beam 14 then propagates in the substrate, down to the lower face 4 of the substrate. A part of the non-guided beam 14 may be reflected by total internal reflection on the lower face 4. A part of the reflected beam 16 may then be transmitted up to the end of the substrate facing the output fiber 30. The output fiber 30 may thus collect not only the guided optical beam 12, but also a part of the non-guided and reflected optical beam 16. FIG. 1 shows only a single reflection on the lower face 4 of the substrate, half the way between the input face 1 and the output face 2, i.e. at the center of the lower face 4. Other multiple internal reflections are also possible.
FIG. 2 shows a sectional view of the integrated optical circuit of FIG. 1, on which is schematically shown the angular distribution of the light power P of the non-guided optical beam in the substrate. The plane of FIG. 2 is defined as being an incidence plane passing through the first end 7 and the second end 8 and perpendicular to the lower face 4. It is observed that a rather high part of the optical beam is optically coupled in the substrate. The non-guided optical wave undergoes a total internal reflection on the upper surface 3. Therefore, the non-guided optical wave is subjected to an interferometric effect of the Lloyd-mirror type on the upper face 3 of the substrate. This results in a Lloyd-mirror interferometer, with interferences occurring between the input fiber 20 and its virtual image. Further, the total internal reflection produces a phase-shift of π. Accordingly, the central fringe of the interferogram, located on the upper face 3, is a black fringe. This explains that the density of power of the non-guided light propagating directly is drastically reduced just below the upper face 3, where the output optical fiber is placed (cf. H. Lefèvre, The fiber optic gyroscope, Artech House, 1992, Annex 3 Basics of Integrated Optics, pp. 273-284). Consequently, a proton-exchange polarizer should have in theory a very high polarization rate of −80 to −90 dB.
However, there exist other couplings of the non-guided optical beam than the direct transmission. Indeed, the substrate may transmit various non-guided beams propagating by internal reflection, in particular on the lower face 4, but also on the upper face 3 or on the side faces 5. Non-guided spurious beams propagating by internal reflection on the faces of the substrate may arrive near a waveguide end 8 on the output face 2 of the substrate.
Generally, the non-guided beams reflected inside the substrate may affect the quality of the signals transmitted in the waveguide of an integrated optical circuit. In the case of a lithium-niobate proton-exchange polarizer, cut following an X plane and comprising an integrated waveguide according to the propagation axis, Y, the guided beam 12 is generally a TE polarization beam and the non-guided beam 14 is a TM polarization beam. Due to the internal reflections of non-guided light in the substrate, the polarization rejection rate of a proton-exchange polarizer according to the schema of FIG. 1 is in practice limited to about −50 dB. Further, the quality of an integrated polarizer influences the performance of certain applications, in particular in an optical fiber gyroscope. It is therefore necessary to improve the rejection rate of an integrated-waveguide polarizer. More generally, it is desirable to improve the optical quality of an integrated optical circuit and to reduce the quantity of non-guided spurious light transmitted by the substrate outside the optical waveguide.
Various solutions have been proposed to solve the problem of spurious coupling of non-guided optical beams between a waveguide input and a waveguide output in an integrated optical circuit.
It is generally admitted that the main contribution to the spurious light comes from the primary reflection of a non-guided beam 14a at a primary reflection point 13a located at the center of the lower face 4 between a first waveguide end 7 on the input face 1 and a second waveguide end 8 on the output face 2. In order to suppress the primary reflection on the lower face of a substrate 4, an integrated optical circuit has been developed, comprising a central groove 25a arranged at the middle of the lower face 4 (cf. the perspective view of FIG. 3 and the top view of FIG. 4). In FIG. 4 is shown the layout of a median plane 17, which is defined as being a plane perpendicular to the lower face 4, perpendicular to a line segment joining the first and the second waveguide ends and which passes through the middle of this line segment. A middle point 18 is defined, which is located at the middle of the line segment joining the first end 7 and the second end 8 of the waveguide 6. The central groove 25a extends over the whole width of the substrate according to a direction perpendicular to the direction of the waveguide 6. However, if a central groove 25a stops the non-guided beam 14a reflecting at the center of the lower face 4 of the substrate, it does not stop the multiple internal reflections occurring between the lower face 4 and the upper face 3. FIG. 5 shows an example of a part of a non-guided optical beam 14b propagating between a first waveguide end 7 and a second waveguide end 8, through double reflection on the lower face and simple reflection on the upper face to form a multiple reflection spurious beam 16b. Therefore, a central groove on the lower face of the substrate allows improving the rejection rate of a proton-exchange polarizer by several orders of magnitude, but the rejection rate remains limited in practice to about −65 dB.
In the case of a Y junction, the U.S. Pat. No. 7,366,372 proposes to arrange a first central groove 25a on the lower face of the integrated optical circuit, half the way between the input face 1 and the output face 2, so as to suppress the primary reflection, and a second central groove 25b on the upper face, arranged between the legs of the Y junction, and half the way between the input face and the output face, so as to suppress the part of the non-guided beam 14b propagating by multiple reflection in the substrate and reflecting on the middle of the upper face (see the sectional view of FIG. 6). However, the central groove 25b on the upper face 3 must not cut the waveguide 6 and is thus limited laterally so as not to cut the legs of the Y junction. This solution is not generalizable to other types of optical integrated circuits.
The U.S. Pat. No. 5,321,779 describes an IOC comprising at least one central groove, extending in the median plane, half the way between the input face and the output face of the IOC, and possibly two side grooves arranged at ¼ and ¾ of the length of the substrate, respectively. According to this document, the effect of the central groove is to attenuate the primary reflection at the central point. As a complement, the effect of the side grooves at ¼ and ¾ of the length is to attenuate multiple reflections between the lower face and the upper face. The grooves at ¼ and ¾ do not affect the primary reflection. However, it is experimentally observed that the presence of side grooves at ¼ and ¾ of the substrate length, on either side of a central groove, reduces only marginally the recoupling of the non-guided spurious light, compared to an IOC having only one central groove.